High reliability radio frequency devices are packaged in hermetically sealed housings. To allow the transmission of radio frequency signals into and out of these housings, hermetically sealed feedthroughs are necessary. These feedthroughs must have proper electrical performance characteristics, while providing a hermetic seal of a reliable nature. Packages with these feedthroughs are used in communications satellites, microwave communications equipment, and military communications and radar systems which require a hermetic seal to avoid contaminating the RF devices inside.
These feedthroughs form a coaxial transmission line through the housing wall and into the cavity containing the RF devices. The coaxial line is comprised of a conductive center pin and a surrounding cylinder of dielectric material. The dielectric material is one of low loss tangent, such as glass. The outer conductor of the coaxial line is formed by the housing wall. To create a better transition of the radio frequency signal to the components within the device, the glass section is followed by an airline section which uses air as the dielectric material.
In order to seal the feedthrough to the RF housing, the feedthrough is heated, causing the glass cylinder to melt and then bond to the housing and the center pin, thus forming a hermetic seal. Unfortunately, when the glass melts, it flows into the airline, thus degrading the electrical performance of the transmission line.
Since a slight gap must be present between the glass cylinder and the housing wall to allow the cylinder to be inserted into the wall, a longer length of glass material is required to ensure that, after melting, the glass completely fills the cylindrical hole. During heating, the glass adheres to the center pin in the area beyond the surface of the housing wall. As the glass contracts into the hole, a meniscus is formed between the connector pin and the outside edge at the housing-glass interface. The curvature of the meniscus reduces the performance of the feedthrough. The meniscus is also brittle and is prone to cracking. It is possible that the cracking of the meniscus can expose base kovan on the feedthrough pin.